Microbiology Study Guide: Skin and Eye Infections (Chapter 17)

Q9. Using the information in TABLE 17.3, Staphylococcus aureus can be differentiated from Streptococcus pyogenes by which two biochemical tests? Hint: S. aureus is positive for these two enzymes, while S. pyogenes is negative.

Background

Topic: Bacterial Identification and Virulence Factors

This question tests your understanding of how to distinguish between two important bacterial pathogens using biochemical tests, specifically focusing on enzyme activity.

Key Terms and Formulas

• Catalase Test: Detects the presence of the catalase enzyme, which breaks down hydrogen peroxide into water and oxygen.

• Coagulase Test: Detects the presence of the coagulase enzyme, which causes blood plasma to clot.

Step-by-Step Guidance

1. Review Table 17.3 to identify which enzymes are present in S. aureus but absent in S. pyogenes.

2. Notice that S. aureus is positive for both coagulase and catalase, while S. pyogenes is negative for both.

3. Understand the principle of the catalase test: When hydrogen peroxide is added to a bacterial sample, catalase-positive bacteria produce bubbles (oxygen gas).

4. Understand the principle of the coagulase test: When plasma is mixed with the bacteria, coagulase-positive bacteria cause clotting.

5. These tests are used in the laboratory to differentiate between S. aureus and S. pyogenes based on their enzyme activity.

Try solving on your own before revealing the answer!

Final Answer:

The two biochemical tests that differentiate S. aureus from S. pyogenes are the catalase test and the coagulase test. S. aureus is positive for both, while S. pyogenes is negative for both.

This distinction is important for clinical diagnosis and treatment decisions.